1. Field of the Invention
This invention relates generally to laying underwater pipelines in relatively deep water using a J-Lay method from a floating vessel and, more particularly, to installing underwater pipelines on the seabed and/or connecting the same to floating terminals at the surface.
2. Description of the Related Art
Subsea pipelines have most often been laid on the ocean floor by connection of welding on the deck of a barge, lowering off the back of the barge down a stinger, and then laying on the ocean floor. The curve down off the barge and then reverse curve onto the ocean floor gives a "S" bend in the pipeline during the lay process. The stinger allows the pipeline and its associated weight to be curved down toward the ocean floor without kinking or damaging the pipeline. As water depths become greater; the size, weight, and cost of the stinger become prohibitive.
In deeper water, it is advantageous to connect the pipeline vertically and lower it directly into the water, with a single bend at the ocean floor. This gives the shape of a "J" for a system known as J-Laying pipe. The "J" cannot be vertical at the top, but rather must have an angle with a horizontal component to be able to pull horizontal tension on the pipeline as it is being laid onto the ocean floor. If it does not have horizontal tension, it will buckle as it is laid on the ocean floor and be damaged beyond use.
Previous J-Lay Towers have lowered the pipe vertically and required that the pipe be bent around a cone shaped stinger to gain the required exit angle, or have had fixed angle towers with the ideal exit angle approximated. Bending the pipe around a stinger from a vertical tower makes the connections difficult and induces a high degree of stress in the pipeline.
The fixed angle towers require that the vessel drive along the pipe route in order to lay the pipeline. In cases where there is a cross wind or cross current, it can be advantageous to turn the vessel into the weather move the vessel somewhat sideways down the pipeline route. In some cases, the vessels will not have the power to hold course when being broadsided by the weather but could hold course if it could face the weather. In other cases, the stability of the vessel is simply better when facing the weather.
Additionally, as water depths and pipeline tension requirements change, the necessary angle of departure of the pipeline can change. Past J-Lay towers have had limited capability at best for changing the angle of the tower to facilitate laying the pipelines.
Rigidly installed, vertical towers engender a number of problems caused by an inability to respond in real time to the dynamic forces encountered during pipe laying operations. For instance, welding and assembly operations are performed at the working floor on pipe with a substantial imposed moment. Other deficiencies include the fact that they do not allow: 1) control of the bending stress and tension within the pipe string as it is deployed in an arc to the sea bed; 2) the laying vessel to weather vane or rotate about the pipe and thereby prevent torsional wind-up of the pipe string; and 3) precise control of the pipe lay envelope. Further, current J-Lay tower designs omit any means for precise and accurate alignment of the pipe string and new pipe joints during the welding process.
Current stinger designs have a problem in that the internal geometry of the stinger is usually fixed at the diameter of the pipe string being deployed. In order to allow the passage of large diameter packages integrated into the pipe string, the stinger must be dismantled or removed from around the hanging pipe string. Additionally, such stingers only act as guide conduits for the pipe and do not stabilize, i.e., control the alignment of, the pipe as it emerges from the weld floor.
An additional alternative to the laying of steel pipelines for this type service has been to lay flexible pipelines. As the pipeline is basically a long pressure vessel, it is inherently obvious that a simple steel tube pipeline would be substantially more economical to manufacture and more reliable than a multi-part flexible pipeline or hose of the same internal diameter and the same high pressure rating. These flexible pipelines have found a place in the market due to the difficulties in the field installation of the lower capital cost steel tube pipeline. The J-Lay Tower attempts to enhance the cost effectiveness of the installation of the steel tube pipeline alternative.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.